1. Field of the Invention
The present invention relates to data detection systems, and more particularly to read amplifiers which sense separate valid data transitions of a data signal from other transitions of the signal.
2. History of the Prior Art
It is commonly known in data processing operations to transmit binary or digital data in the form of a signal having transitions between two different levels such that the transitions represent the data. Data signals of this type may, for example, comprise a magnetic recording in which transitions between opposite levels of magnetic saturation represent the data being transmitted. A number of encoding formats have been devised for carrying data in this fashion. One of the better known formats is so-called phase encoding in which the sense, polarity of direction of the transitions at particular time locations along the length of the data signal represent the information being carried.
In reading the data carried by data signals employing phase encoding and certain other types of encoding, it is generally necessary to both detect the transitions and their sense and determine the location of the transitions relative to a time scale. For example in the case of phase encoding the data signal is arbitrarily divided into a succession of bit intervals. The senses of transitions at the beginnings or leading edges of the bit intervals denote the data. However, further transitions appear in the region of the centers of various ones of the bit intervals since the polarity of the signal must reverse where the immediately following data transition is to have the same sense as the preceding transition. Accordingly it is necessary to provide the read amplifier with timing circuitry for defining the bit intervals so that it can be determined whether a sensed transition represents a data signal or merely a polarity reversal in the signal.
A number of techniques are commonly employed for providing a time reference in connection with a data signal having a succession of transitions. One of the more common techniques involves the use of a phase locked oscillator. The oscillator is locked to the data signal to define the various bit intervals or other time reference. However, a number of disadvantages commonly arise from conventional arrangements of this type. For one thing phase locked oscillating arrangements tend to be relatively complex. Moreover, such arrangements do not lock onto the data signal to the extent required for acceptable accuracy in many applications. Typically such arrangements do not respond to every leading edge transition but only to selected transitions. As a result the oscillator provides a uniform signal defining a succession of uniform bit intervals. As a practical matter, however, data signals tend to be non-uniform. For one thing such signals can vary significantly from one bit interval to the next, leading to errors where a phase locked oscillator arrangement is used. In those arrangements where the bit intervals are arbitrarily defined based on given conditions at the beginning of operation, further errors can occur, for example, simply due to variations in the speed of the medium carrying the data signal relative to the transducer.